it's recommended to launch gta 5 enhanced at least once (it will make save file, so it's easier to find it)
gta 5 legacy save game location:
you should have only 1 folder there, which will have some random numbers and/or lettersDocuments\Rockstar Games\GTA V\Profiles
if you have more, then you have to test and see which is which.
reason why you might have more than 1, is if:
+ you have played gta v with more than 1 account/profile from same pc
+ you have used different version of gta v (cracked copy or other platform/s)
enhanced version should have same folder name/numbers, but if it doesn't, then use common sense to find which one is correct save (check time stamps, save size ..etc)
copy correct save folder (just press copy on it, don't actually move/copy it anywhere specific yet).
GTA V enhanced savegame location:
compare the folder found there vs legacy save game name/sDocuments\Rockstar Games\GTAV Enhanced\Profiles
one with matching name is your profile/account (if you have more than 1)
* you can make a backup of original save by just renaming it, like 12345GTA to 12345GTA-backup. if you don't want the backup, then delete the save
now paste the legacy save folder into that location
DO NOT overwrite the previous folder, it's important that you will DELETE it or RENAME it before pasting legacy save, because there might be files that have different names so they don't get overwritten and it might conflict.
that's it, launch enhanced version and you will have your save.
for online save, just go to online mode and it will ask if you want to convert / merge the old save with new, those actions are server-sided and CANNOT be UNDONE! it's highly recommended (specially to drooling button bashing rushers) NOT TO bash buttons and NOT TO rush, i know drooltards every instinct tells them to bash buttons hard and drool while doing so, BUT DON'T! if you accidentally rush / bash your way past the "convert old save" then that's it, your old save will be gone forever as it's stored on server, not on your PC.